Preheat your oven to 400°F (200°C). Scrub the sweet potatoes clean, then prick them a few times with a fork. Roast directly on a baking sheet for 45–50 minutes, until very tender and caramelized on the edges.
While the potatoes bake, rinse the quinoa under cold water. In a small pot, combine quinoa and water or broth. Bring to a simmer, cover, and cook for about 15 minutes, or until the quinoa is fluffy and the liquid is absorbed. Remove from heat and let it sit, covered, for 5 minutes.
In a large bowl, mash the warm roasted potatoes slightly to create a creamy interior that will lightly blend with the quinoa. Fluff the quinoa with a fork and add it to the bowl.
Add the lemon juice, minced garlic, chopped parsley, oregano, and a pinch of salt and pepper. Fold in cherry tomatoes, cucumber, chickpeas, feta, olives, and a drizzle of olive oil. Mix gently to combine, keeping some texture for a pleasant bite.
Taste and adjust seasoning. If you like a touch more brightness, add a little extra lemon juice. If you want more creaminess, blend in a spoonful of yogurt or tahini at the end.
Spoon the quinoa mixture into the roasted potato boats, dividing the filling evenly. Return to the oven for 5–7 minutes to let the flavors meld and the tops warm through.
Remove from the oven and let rest for a couple of minutes before serving. Garnish with extra parsley and an optional extra drizzle of yogurt or tahini.